Ventricular fibrillation is a major cause of sudden death. Although the efforts of Cobb and others who have aggressively pushed energency resuscitation of patients developing out-of-hospital ventricular fibrillation have yielded impressive results, the vast majority of such patients still are not being successfully resuscitated. Considerable efforts to develop antiarrhythmic drugs for the prevention of ventricular fibrillation have so far been only partially successful and the near-future success of such effort is not assured. The objective of the research effort proposed here is to contribute to the development of a standby automatic implanted ventricular defibrillation system which will be suitable for use in patients who are at high risk of experiencing out-of-hospital episodes of ventricular fibrillation. Patients who have survived an out-of-hospital episode of ventricular fibrillation, who have not had a recent transmural myocardial infarction, and whose angiographic findings indicate a poor prognosis are at very high risk of suffering recurrent ventricular fibrillation and constitute a population which should be the first to benefit from the availability of automatic implanted defibrillators. During the period covered by the present application, we intend to: (a) Make an intensive effort to identify and evaluate power handling and energy storage components which will meet the unusual specifications of the implanted defibrillator and have the best available power to weight and energy to weight ratios. Considerable long-term reliability testing is projected. (b) Extend our earlier studies concerning the defibrillatory energy requirements of various implanted electrode systems to include epicardial and epicardial-catheter systems. This will include acute studies in both dogs and calves as well as long-term chronic studies in dogs. (c) Design and fabricate defibrillators on the basis of the results obtained in (a) and (b). Complete systems will be evaluated by means of long-term studies in calves.